If I earn a D or F in a class do I have to retake it? If I retake it does it replace my old grade?

Earning a D will fulfill your graduation requirements, however four year universities require a C or better and so retake is recommended. If you earn an F, you will have to retake the class if it is required for graduation.
If you retake the class and achieve a better grade, your old grade will still appear on your transcript but there will be a repeat symbol to indicate that you retook the course. However, the lower grade will no longer be accounted for in your grade point average (GPA).

Haddee Team
Education with Inspiration